1. Field of the Invention
The present invention relates to a composite marking based on chiral nematic (also called cholesteric) liquid crystal precursors and in particular, a marking wherein a modifying resin changes the position of the selective reflection band exhibited by a cured chiral liquid crystal precursor composition in the chiral liquid crystal state. The invention also relates to a method of providing a substrate with the composite marking and to a method of changing the selective reflection band exhibited by a cured chiral liquid crystal precursor composition on a substrate.
2. Discussion of Background Information
Counterfeit is no longer a national or a regional problem but a worldwide problem which has an impact not only on manufacturers but also on the consumer. Counterfeiting is a significant problem with goods like clothes and watches but becomes even more serious when it affects medicines and drugs. Each year thousands of people around the world die because of counterfeit drugs. Counterfeiting has also an impact on government revenues in that it affects the collection of taxes for, e.g., cigarettes and alcohol because of the existence of a black market where it is impossible to track and trace counterfeit (smuggled, diverted, etc.) products with no valid tax stamps.
Many solutions have been proposed to make counterfeiting impossible or at least very difficult and/or costly, for example RFID solutions and the use of invisible inks.
More recently, a security feature has emerged and is used to authenticate a genuine product such as a drug and to avoid counterfeiting. This technology is based on optically variable inks. Its principle is based on the difference in observable color of a marking made with optically variable inks when a packaging, security document, etc. carrying the marking is viewed from different angles (“viewing-angle dependent color”).
Optically variable inks provide first-line recognizability not only by a person, but also facilitate machine-readability. Many patents describe this security product, its composition and its application. One example of the many types of optically variable inks is the class of compounds called cholesteric liquid crystals. When illuminated with white light, the cholesteric liquid crystal structure reflects light of a certain color which depends on the material in question and generally varies with the viewing angle and the temperature. The cholesteric material itself is colorless and the observed color is the result of a physical reflection effect at the cholesteric helical structure that is adopted by the liquid crystal precursor composition at a given temperature. See, e.g., J. L. Fergason, Molecular Crystals, Vol. 1, pp. 293-307 (1966), the entire disclosure of which is incorporated by reference herein.
EP-A-1 381 520 and EP-A-1 681 586, the entire disclosures of which are incorporated by reference herein, disclose a birefringent marking and a method of applying the same in the form of a liquid crystal layer having a non-uniform pattern of regions of different thickness. The applied liquid crystal coating or layer may provide for a hidden image on a reflected substrate, which image is invisible when viewed under non-polarized light but is rendered visible under polarized light or with the help of a polarization filter.
U.S. Pat. No. 5,678,863, the entire disclosure of which is incorporated by reference herein, discloses means for the identification of documents of value which include a paper or polymer region, said region having a transparent and translucent characteristic. A liquid crystal material is applied to the region to produce an optical effect, which differs when viewed in transmitted and reflected light. The liquid crystal material is in liquid form at room temperature and must be enclosed in a containing means such as microcapsules in order to be suitable for use in a printing process such as gravure, roller, spray or ink-jet printing.
The ordered liquid crystalline state depends upon the presence of a chiral dopant. Nematic liquid crystals without chiral dopant show a molecular arrangement that is characterized by its birefringence. Nematic polymers are known from, e.g., EP-A-0 216 712, EP-A-0 847 432, and U.S. Pat. No. 6,589,445, the entire disclosures of which are incorporated by reference herein.
As mentioned above, the liquid crystal based security feature provides first-line recognizability by the consumer and also by retailers and producers of goods and articles. Like for many other security features which are used in the market, there is always the temptation of counterfeiters to reproduce these security features and therefore misleads consumers and retailers. In view of the foregoing facts, there continues to be a need to improve the security of liquid crystal polymer materials based on liquid crystal precursors.
One possibility of enhancing the security level of a chiral liquid crystal polymer film would appear to be superposing a code in the form of a pattern, indicia, a bar code, etc. on the liquid crystal polymer film. However, there is always a risk that a counterfeiter tampers with the code and manually applies it onto the liquid crystal polymer film.
A second possibility of overcoming this problem is to insert the code inside the liquid crystal polymer film. For example, U.S. Pat. No. 6,207,240, the entire disclosure of which is incorporated by reference herein, describes an effect coating of a cholesteric liquid crystal polymer (CLCP) with viewing angle dependent reflection color that further comprises absorption type pigments exhibiting a specific absorption color. A marking, such as a symbol or a text, is generated in the CLCP coating by laser irradiation. The laser radiation carbonizes the CLCP material in the irradiated area. As a result, the color of the substrate on which the CLCP is coated, or the color of absorption pigments incorporated into the CLCP, becomes visible in the irradiated area. However, the method requires high-power lasers to carbonize the material and to make the markings visible.
Another possibility is described in US 2006/0257633 A1, the entire disclosure of which is incorporated by reference herein, which is applied not only to liquid crystal polymers but to polymers in general. The method consists of applying a permeating substance to a predetermined region on the surface of the polymer substrate and bringing a supercritical fluid into contact with the surface of the polymer substrate to which the permeating substance has been applied to cause the permeating substance to permeate into the polymer substrate. The method makes it possible to selectively (partially) modify a portion of the surface of the polymer. However, for industrial processes where a high marking speed for a large number of items is required the method is complex and expensive to implement.
One of the drawbacks of the methods cited above is the lack of ability to modify the chiral liquid crystal polymer layer in a selective and controlled manner and to create a strong and reliable marking or coding that is difficult, if not impossible, to reproduce and also is compatible with a production line (processes for making items such as passports, packaging, etc.).